The invention relates to a circuit configuration for modulation with a transmission PLL. Such modulators are particularly suitable for GMSK modulation in mobile radios.
In mobile radios operating in the GSM Standard, data are transmitted using GMSK (Gaussian minimum shift keying) modulation. At the moment, the same type of modulation is used both for voice transmission and for data transmission. However, the data rate is not sufficient for multimedia applications going beyond a pure voice service, so that a more recent standard proposes 8-PSK modulation. 8-PSK modulation allows three times as much data to be transmitted per unit time. The GMSK symbols have a constant envelope curve, that is to say a constant amplitude, while the 8-PSK symbols have a non-constant envelope curve, and have a different amplitude. According to the specification in the Standard, it is possible to switch directly between GMSK modulation and 8-PSK modulation from one time slot to the next. The combined transmission system is called GSM Edge.
A transmission PLL (PLL: Phase Locked Loop) or Up-conversion Loop is used for GMSK modulation. The literature reference G. Irvine et al.: xe2x80x9cAn Up-conversion Loop Transmitter IC for Digital Mobile Telephonesxe2x80x9d, 1998 IEEE International Solid-State Circuits Conference, pages 364-65, describes a transmission PLL. A voltage-controlled oscillator (VCO) is actuated by the phase information and produces the modulated output signal for transmission via the antenna, after appropriate amplification. The VCO output is fed back via a down-conversion mixer, to which the output signal from a channel oscillator is supplied, and via a quadrature component modulator to a phase detector in the phase locked loop. The downward-modulated signal is compared with a reference frequency in the phase detector. The output signal from the phase detector is passed via a loop filter in order to produce the frequency control signal for the VCO. In Irvine et al., the quadrature component modulator is actuated by the downward-mixed output signal. The literature reference Yamawaki et al.: xe2x80x9cA 2.7-V GSM RF Transceiver ICxe2x80x9d, IEEE Journal of Solid-State Circuits, Volume 32, No. 12, December 1997, page 2089-96 describes a transmission PLL in which the output of the down-conversion mixer is fed back without being modulated to one input of the phase detector, and the other input of the phase detector is fed from an oscillator, onto which the quadrature components of the wanted signal are modulated. The quadrature component modulator is located upstream of the phase discriminator, either in the feedback loop of the transmission PLL (Irvine et al.) or in the reference signal phase (Yamawaki et al.).
The loop filter of the transmission PLL is a low-pass filter. At radio frequencies it acts as a narrowband bandpass filter within the transmission PLL, in order to allow the wanted signal to pass. The VCO is of low-noise design. This solution has been adopted in particular because there is no need for any duplex filter downstream of the transmission amplifier and immediately upstream of the antenna. The duplex filter ensures that the transmission signal is attenuated outside the transmission band, and prevents noise and nearby transmissions from being injected into the reception channel. It is relatively expensive and necessitates increased power consumption in the transmission amplifier.
Owing to these advantages, it is desirable to continue to use the concept of the transmission PLL in appliances based on the combined GMSK/8-PSK Standard as well. Since the amplitude of the output signal from the VCO is constant in GMSK modulation, but, in contrast, 8-PSK modulation has a variable-amplitude envelope, it is not possible to use a transmission PLL for 8-PSK modulation without some further action.
The object of the invention is to provide a modulator circuit assembly which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which is suitable for two modulation types, in particular GMSK and 8-PSK modulation.
With the above and other objects in view there is provided, in accordance with the invention, a modulator circuit configuration, comprising:
a first modulator having
a controllable-frequency oscillator with an output for a first output signal;
a down-conversion mixer connected to the output;
a phase detector having a first input and a second input receiving the first output signal of the oscillator fed back via the down-conversion mixer; and
a mixer device configured to produce a signal modulated with a wanted signal;
a second modulator having
an input connected to the output of the oscillator; and
a mixer device configured to produce a signal, modulated with the wanted signal, as a second output signal;
a first switching device connected to the first and second modulators for switching between the first output signal and the second output signal; and
a second switching device connected to and feeding an unmodulated signal to the first input of the phase detector when the first switching device is switched to the second output signal, and feeding the signal modulated with the wanted signal to the first input of the phase detector when the first switching device is switched to the first output signal.
In other words, the objects of the invention are achieved by the circuit configuration for modulation which comprises: a first modulator having a controllable-frequency oscillator which has one output for a first output signal; a phase detector in which the output of the oscillator is fed back via a down-conversion mixer to the phase detector, and a mixer arrangement in order to produce a signal which is modulated with a wanted signal; a second modulator which is connected on. the input side to the output of the oscillator and which comprises a mixer arrangement in order to produce a signal, which is modulated with the wanted signal, as a second output signal; a first switching device in order to switch between the first and the second output signal, and a second switching device in order to produce an unmodulated signal at one input connection of the phase detector when the first switching device is switched to the second output signal, and in order to produce the signal which is modulated with the wanted signal at the input connection of the phase detector when the first switching device is switched to the first output signal.
In the circuit arrangement according to the invention, a transmission PLL is still used for GMSK modulation, and the advantages obtained from this are retained. When 8-PSK modulation is required, the arrangement switches to a direct modulator. The VCO in the transmission PLL is used as the oscillator for the carrier frequency of the direct modulator. For this purpose, the quadrature component modulator in the transmission PLL is switched to be inactive. The arrangement according to the invention thus combines the principles of the transmission PLL and of direct modulation, with particularly complex or expensive components being used jointly.
Owing to the lower specific transmission power for 8-PSK modulation, the standardization envisages less stringent noise requirements for the transmission path. Corresponding to the transmission power, the signal-to-noise ratio at 10 MHz and 20 MHz from the transmission carrier is around 6 dB lower than for GMSK modulation. Filtering is required after the power amplifier for GMSK modulation, since the noise in the transmission band is convoluted into the receiver band by the nonlinear amplifier (noise conversion gain). When a linear amplifier is used for 8-PSK , this effect occurs only to a very weak extent. It is thus sufficient to provide a surface acoustic wave filter at the output of the direct modulator and upstream of the transmission amplifier, in order to avoid transmission into the reception channel. There is no longer any need for a duplex filter, which would be required if a direct modulator were to be used for GMSK modulation, in order to comply with the requirements of the GSM edge Standard for crosstalk attenuation for 8-PSK modulation. The GMSK path does not require any duplex filter, so that the speech time of the mobile phone is further improved.
The use of the VCO in the transmission PLL as a synthesizer for the direct modulator provides a low-noise input signal for the direct modulator. The high-speed-regulating synthesizer with the relatively broad loop filter ensures that load-pulling effects, which can occur in comparison to conventional channel synthesizers at a limit frequency, are avoided in the arrangement according to the invention.
For 8-PSK operation, the VCO in the transmission PLL is expediently operated in an economy mode, so that the available mobile radio speech time is increased further.
In accordance with an added feature of the invention, there are provided terminal connections receiving quadrature components of the wanted signal, a switching device connected to the terminal connections, and further connections for carrying a constant signal, and wherein:
the switching device is connected and configured such that the wanted signal can be supplied to the mixer device of the second modulator, and the second switching device is connected and configured such that the constant signal can be supplied to the second mixer device when the first switching device is switched to the second output signal; and
the second switching device is connected and configured such that the wanted signal can be supplied to the mixer device of the first modulator when the first switching device is switched to the first output signal.
In accordance with an additional feature of the invention, there is provided a further oscillator, and the second switching device is connected and configured such that an input of the mixer device of the first modulator is coupled to the further oscillator, and an output of the mixer device of the first modulator is coupled to the first input of the phase detector when the first switching device is switched to the first output signal; and
the first input of the phase detector is directly connected to the further oscillator, bypassing the mixer device of the first modulator, when the first switching device is switched to the second output signal.
In accordance with another feature of the invention, a nonlinear amplifier is connected between the oscillator and the first switching device for coupling the first output signal to the first switching device, and a linear amplifier is connected to couple the second output signal to the first switching device.
In accordance with a further feature of the invention, there is provided an amplifier that can be switched between a first operating mode with nonlinear amplifier operation and a second operating mode with linear amplifier operation, and a switch connected to selectively supply the first output signal or the second output signal to an input of the amplifier.
In accordance with again an added feature of the invention, a surface acoustic wave filter is connected downstream to an output of the mixer device of the second modulator.
In accordance with again an additional feature of the invention, the mixer device of the first modulator is connected to an input of the down-conversion mixer.
In accordance with again another feature of the invention, the oscillator has an operating mode with a high driver capability and an operating mode with a lower driver capability, and wherein the second operating mode is selected when the first switching device is switched to the second output signal.
In accordance with a concomitant feature of the invention, the wanted signal is supplied with quadrature components, and each of the modulators includes:
a phase element for splitting a carrier signal into signal components phase-shifted through 90xc2x0 with respect to one another;
two mixers each receiving one of the signal components and one of the quadrature components; and
an adder element connected to the two mixers for adding output signals from the two mixers.
Various alternative concepts may be used for switching between the transmission PLL and the direct modulator. The quadrature component modulator in the transmission PLL is expediently actuated with constant quadrature signal components during 8-PSK operation, so that, effectively, there is no modulation. The additional complexity for providing the circuit arrangement according to the invention then comprises a further integrated circuit, which contains the components of the direct modulation arrangement. This circuit is compatible with, and can be combined with, previous integrated circuits for the transmission PLL. It is possible to continue to use previous circuit architectures just by adding the integrated circuit for the direct modulator, in order to develop GMSK-compatible mobile telephones further for combined GMSK/8-PSK operation. Alternatively, the quadrature component modulator in the transmission PLL can be bypassed for 8-PSK operation. The modulator is expediently switched off in this case, thus saving power consumption. However, this concept requires the integrated circuit for the transmission PLL to be redesigned.
The output stage amplifiers for the transmission PLL operate in a known manner nonlinearly with a limiter characteristic, since the GMSK symbols have a constant envelope curve or amplitude. The transmission output stage for the direct modulator in 8-PSK operation requires an amplifier with a linear transmission characteristic, since the envelope curve of the 8-PSK symbols or their amplitude is variable, and is thus not constant. The transmission paths of the transmission PLL and the direct modulator can on the one hand be combined downstream from the transmission amplifiers, in which case a nonlinear amplifier stage can be provided for the transmission PLL, and a linear amplifier stage can be provided for the direct modulator. On the other hand, the transmission paths can be combined upstream of the power output stage, in which case the amplifier can be designed to have a switchable operating point, so that nonlinear and linear amplifier operation can be carried out by a single amplifier stage as a function of the respective modulation type.
The transmission architecture according to the invention makes it possible to provide a solution optimized in terms of power consumption and production costs both for GMSK operation and for 8-PSK operation. Compatibility can be ensured with previous integrated transmission PLL circuits. This results in effective double utilization of components. The system can be extended without any problems to dual-band mobile radios, which comply with both the GSM Standard and the PCN Standard.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a modulation arrangement, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.